There are only 2 black wires, unlabled. I have not disconnected anything yet. Where do they go on the new programmable thermostat: G, RC, RH, Y, or W?

Only two wires indicate a heat only situation. Read your manual that came with the thermostat BUT most of the time it will be RH and W for heat only. If the thermostat uses batterys make sure a fresh set is in the thermostat. I also hope this is a 24 volt system you are working on if it is a millivolt system then problems could happen since you did not post the model number of the new thermostat you are using.

See this link for a previous discussion on this thermostat replacement.

https://www.askmehelpdesk.com/heatin...at-426147.html

Thanks. Yes the new one is compatible with a millivolt system, which is what I needed. So it doesn't matter which goes in the Rh and the W, just as long as there's a wire in each one?

I have a replacement that is compatible with millivolt systems and I have been told that the two available wires go in the Rh and W terminals. Does it matter which one goes in the Rh and the W, or as long as there's a wire in each one I'm okay?

Yes

Thanks hvac1000, everything works great and you saved me at least $85which is the avg. service call around here. It's nice that there are folks like you willing to offer good advice free of charge.

Thanks for the kind words. I am just one of the folks around here that tries to help out. Glad you got it up and running. Keep warm.